1. Field of the Invention
The present invention relates to a method and apparatus for trapping charged particles such as electrons or ions in a particular space by an electromagnetic means.
2. Description of the Prior Art
FIGS. 1A-1C illustrate conventional apparatuses for trapping charged particles in a space: FIG. 1A shows a conventional quadrupole mass spectrometer; FIG. 1B shows a Paul trap for three-dimensionally confining charged particles on the same principle as in FIG. 1A; and FIG. 1C shows a Penning trap that operates on the basis of an electric field and a magnetic field which are orthogonally applied.
In FIG. 1A, four trapping electrodes 31-34 surround a space in which charged particles are to be stably contained. Two opposing electrodes 31 and 33 of the four electrodes are supplied with a radio frequency voltage from an alternating current power supply 2 as shown in FIG. 1A so that a radio frequency electric field is produced in the space. This apparatus two dimensionally traps charged particles in the space surrounded by the electrodes 31-34 by utilizing the principle that the time averaged force that acts on the charged particles placed in the space directs such a direction as the gradient of the electric field reduces, when the radio frequency voltage is applied to the electrodes 31 and 33.
The apparatus of FIG. 1B three dimensionally traps charged particles in a space surrounded by trapping electrodes 35 and 36 and an annular electrode 37 whose surfaces are formed by a hyperboloid of revolution. The device of FIG. 1B traps charged particles on the same principle as that of FIG. 1A.
As clearly seen from the principle, the gradient of the electric field in the trapping space where the charged particles are to be confined must be made smaller than that around the space. Thus, the periphery of the trapping space must be surrounded compactly by the electrodes. The trapping principle of FIG. 1A is sensitive to the mass of the charged particles, as is apparent from the fact that the arrangement of FIG. 1A is extensively used as a mass spectrometer. This is because the trapping is achieved by oscillating the charged particles by the radio frequency electric field, and hence, no trapping force acts on particles of a great mass because of their small oscillation amplitude. In contrast, light particles collide against the electrodes 31-34 because of their large oscillation amplitude.
In FIG. 1C, the same electrodes as those of FIG. 1B are supplied with a voltage from a direct current power supply 3 so that charged particles are trapped in the z direction, and are diverged in the horizontal direction (or the r direction of FIG. 1C). At the same time, a magnetic field 39 in the z direction is generated by a pair of magnets 38. The direct voltage causes the charged particles to move away from the center in the horizontal direction, whereas the magnetic field 39 deflects the course of the movement so as to prevent the charged particles from running away. Since the force acting on the particles is proportional to the velocity of the particles, the trapping effect on heavy particles by the magnetic field 39 reduced even if the applied voltage to the electrode is maintained constant. Thus, the mass of particles that can be trapped is limited to a narrow range.
The conventional methods described above present the following problems:
(1) The mass of particles that can be trapped is limited to a narrow range. PA1 (2) In the Penning trap as shown in FIG. 1C, if the charged particles lose their energy by scattering due to residual gas in a vacuum vessel, or the like, they move away from the center in the radial direction, and hence, escape from the trapping space or collide with the electrodes 35 and 37. As a result, this method cannot hold the particles in suspension for a long time. In addition, trapping heavy particles such as heavy ions requires an extremely large magnetic field, and therefore, the mass and energy that can be trapped are limited to small values. Thus, the method can be applied only to electrons or some light ions. PA1 (3) In the methods of FIGS. 1A and 1B, the trapping space must be surrounded by the electrodes. This not only hinders the charged particles from entering the trapping space, but also hinders an electron beam or a light beam for measurement from being introduced into the space. Furthermore, in the conventional apparatuses, since the trapping space is surrounded with electrodes and such a trap must be performed in vacuum, there arises another problem in that the efficiency of raising the vacuum level of the trapping space reduced because of low evacuation conductance due to the electrodes surrounding the trapping space. Moreover, since the force for confining the particles is generated by the radio frequency electric field, the effective force for trapping becomes relatively small compared with the applied voltage. Therefore, the conventional apparatuses present a problem in that not only is the energy of the trapped particles limited, but also the density of the particles is low because the trapping force cannot oppose the electrostatic repulsion among the particles. In addition, there is another problem in that the charged particles cannot be trapped unless they are synchronized with the phase of the radio frequency voltage for the trap when they are led into the trapping space from the outside. PA1 generating a DC electric field exerting on the charged particles forces directing to a central electrode; and PA1 generating an AC electric field declining its intensity with an increase in the distance from the central electrode in such a manner that the DC electric field and the AC electric field are super imposed. PA1 a central electrode; PA1 an AC voltage source connected to the central electrode for generating an electric field declining its intensity with an increase in the distance from the central electrode; and PA1 a DC voltage source connected to the AC voltage source for generating an electric field exerting on the charged particles forces directed to the central electrode in such a manner that the DC electric field and the AC electric field are superimposed. PA1 a central electrode; PA1 an AC voltage source connected to the central electrode for generating an electric field declining in its intensity with an increase in the distance from the central electrode; PA1 a DC voltage source connected to the AC voltage source for generating an electric field exerting on the charged particles forces directed to the central electrode in such a manner that the DC electric field and the AC electric field are superimposed; PA1 an outer electrode provided outside the central electrode for defining motion of the charged particles; PA1 an evaporation source provided outside the outer electrode for emitting a beam of ions and neutral atoms of an evaporation material toward the central electrode; and PA1 a shutter admitting or blocking the beam transmitting toward the central electrode.
Thus, although the apparatuses of FIGS. 1A-1C have been employed for the purpose of mass spectrometry or trapped ion spectrometry, they have problems and restrictions when used for other purposes. In particular, in order to apply an electromagnetic trap method to charged particles for a material procedure, which treats materials held in suspension in a completely separated state from the inner walls of a vessel, such as crystal growth performed in a suspended state in space, a trap method is required that can trap particles a) at the outside of electrodes, b) in a wide mass range, c) in a wide energy range of particles, and d) with a high density.